1. Field of the Invention
The present invention relates generally to mixing chambers for mixing gases. More particularly, the invention concerns a burner mixing chamber for mixing air and a combustible gas to form a combustible mixture.
2. Discussion of the Invention
In the past a number of approaches to the mixing of air and combustible gases such as natural gas and propane have been suggested. These include the use of cast iron burners, perforated, ribbed metal burners and numerous variations of the Venturi principle to supply an air-gas mixture for combustion.
A common prior art burner design involves the use of perforated, ribbed metal burners which are provided with a number of small venturi orifices, each having a single nozzle with an orifice to supply the combustible mixture. For typical heating requirements several burner units, each with a ribbed or slotted metal burner surface, are assembled together and held in position by various mechanical expedients. Spacing between the venturi orifices permits secondary air to be supplied to the flame for good combustion. Typical secondary air flow is on the order of 0 to 25 percent of the total air supply flowing to the inlet of the venturi orifices. A major drawback of these types of prior art burners is that, because of relatively inefficient combustion, they produce high levels of undesirable emissions such as carbon monoxide and nitrous oxides.
Another common prior art burner design which is based on the venturi principle, involves the inspiration of air into the venturi inlet of a single or double venturi by the flow of a higher pressure gas in a manner to create a negative absolute pressure at the entrance of the venturi inlet. Typically, a separate the venturi is placed into a metal chamber over which the burner material is superimposed with the air-gas mixture being ignited at the burner surface. In this type of design, the pressure drop experienced across the burner surface generally prevents good quality combustion due to insufficient inspiration of air and relatively high emission levels are common. In negative absolute pressure applications, this latter type of burner design is less than desirable and a reasonable flame stability is most difficult to achieve.
The burner mixing chamber of the present invention overcomes many of the drawbacks of the prior art systems by providing a mixing chamber of unique configuration that enables precise mixing of the input gas and sufficient air to produce a gas mixture that burns efficiently with surprisingly low emission levels. As will be better understood from the description which follows, in the apparatus of the present invention the inspirator is an integral part of the burner mixing chamber and the apparatus does not rely on secondary air to enhance combustion. The apparatus includes a combustion chamber of novel design that eliminates the costly spun metal, three dimensional venturi designs typically found in prior art systems and combustion at the burner surface is extremely stable with air in excess of stoichiometric air-gas ratios being readily attained. Large variations in excess air, for example, 10 to 100 percent, are possible without sacrificing combustion stability. Similarly, variations in gas flow due to pressure changes, gas heating value and like effects are readily accommodated and high quality combustion with very low carbon monoxide and nitrous oxide emissions is consistently achieved.